Many important natural products are high value-added commodities which are produced industrially on relatively small scale. For example, .beta.-carotene is produced at a level of approximately 45,000 kg per year, generating between 80 and 100 million dollars in annual sales. There are many important applications of .beta.-carotene, the most significant of which is its action as the most potent vitamin A precursor in the human body. In this respect, it is employed extensively as a nutritional supplement. .beta.-carotene is also employed as a safe food and drug colorant, and has therapeutic uses in the treatment of several skin disorders. Most of the worldwide production of .beta.-carotene is by synthetic means, although carotenoids are produced in nature at a level of approximately 10.sup.8 tons per year by plants and vegetables.
The high cost of synthetic .beta.-carotene and the relative abundance of natural .beta.-carotene makes .beta.-carotene an ideal candidate for supercritical fluid extraction from natural sources. The use of natural products extracted using nontoxic solvents such as supercritical carbon dioxide has become very attractive during the last decade. The extraction of caffeine from coffee is an example of a successful application of this technology on an industrial scale.
While there have been many laboratory successes in supercritical extraction, scale-up to the commercial level has been slow for a variety of reasons. The control of the extraction process, which depends on solvent-solute phase equilibrium, is extremely difficult. Favorable designs often require operation in tightly constrained regions of the phase diagram. In addition, the natural product solutes of interest, such as .beta.-carotene, can be very unstable and are subject to degradation by light, heat and oxidation.
One economical process design, based on minimum utility cost and minimum annualized cost, which has been developed for extraction of .beta.-carotene using supercritical carbon dioxide is depicted schematically in FIG. 1. One serious shortcoming of this process concerns the separator which comprises a simple flash tank. First, the residence time in the flash tank is very high and can lead to chemical decomposition of the extracted .beta.-carotene. Second, the product can contain up to 20 percent water, whereby an additional drying step, which may make the entire process uneconomical, is required. Third, even if the drying process may be done economically, the resulting .beta.-carotene powder is itself very unstable, and must be cooled to -40.degree. C. for storage. These problems are encountered in the supercritical extraction of many other natural products as well since many such compounds have similar instabilities. An additional disadvantage is encountered in the flash tank separator employed in the method of FIG. 1. Since the product is a solid, the separator shown in FIG. 1 cannot operate in a true continuous mode, and the product must be removed from the separator in a semi-batch mode using lock-hopper techniques.
Additional extraction methods employing carbon dioxide under supercritical conditions are disclosed in the Schwengers U.S. Pat. No. 3,939,281, the Schultz U.S. Pat. No. 3,966,981 and the Schutz et al U.S. Pat. No. 4,632,837. The methods disclosed in these references generally teach that the extracted products are precipitated as liquids and the low boiling carbon dioxide solvent is converted to the gaseous state and recycled for further extraction use. The Vitzthum et al U.S. Pat. Nos. 4,123,559 and 4,198,432 disclose processes for the production of spice extracts by extraction with a supercritical gas such as carbon dioxide. The essential oils are withdrawn by treating the spices with dry, supercritical carbon dioxide while the flavor components are withdrawn by treating the spices with wet, supercritical carbon dioxide. Additional supercritical and related extraction methods are disclosed in the Laws et al U.S. Pat. Nos. 4,212,895 and 4,218,491, the Shimshick U.S. Pat. No. 4,250,331, the Wheldon et al U.S. Pat. No. 4,282,259, the Katz U.S. Pat. No. 4,472,442, the Friedrich et al U.S. Pat. No. 4,493,854, the Stahl et al U.S. Pat. No. 4,548,755, the Shindler U.S. Pat. No. 4,601,906, the Traitler et al U.S. Pat. No. 4,703,060 and the Pennisi et al U.S. Pat. No. 4,714,526. However, none of these cited references provide a teaching which overcomes the disadvantages noted above encountered in the separation of desired natural products such as .beta.-carotene from the supercritical fluid extraction solution.